Dynamic taxonomy process for browsing and retrieving information in large heterogeneous data bases

ABSTRACT

A process is disclosed for retrieving information in large heterogeneous data bases, wherein information retrieval through visual querying/browsing is supported by dynamic taxonomies; the process comprises the steps of: initially showing (F1) a complete taxonomy for the retrieval; refining (F2) the retrieval through a selection of subsets of interest, where the refining is performed by selecting concepts in the taxonomy and combining them through boolean operations; showing (F3) a reduced taxonomy for the selected set; and further refining (F4) the retrieval through an iterative execution of the refining and showing steps.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of and claims the benefitof priority from Ser. No. 10/819,946, filed Apr. 8, 2004, which claimsthe benefit of application Ser. No. 09/868,339, filed Jun. 18, 2001,which claims the benefit of PCT application PCT/IT99/00401/, filed Dec.3, 1999, and prior Italian Patent Application No. TO98A 001049, filedDec. 16, 1998. The contents of each of the above applications areincorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention refers to a dynamic taxonomy process for browsingand retrieving information in large heterogeneous data bases.

Information retrieval on this type of database (for example thoseavailable on the Internet) is nowadays a slow task, sometimes impossibleto realize due to the enormous amount of data to be analyzed, and thatcan be implemented with difficulty with the currently available tools.The following documents deal with the prior art in this field: Hearst M.et al: “Cat-a-cone: an interactive interface for specifying searched andviewing retrieval results using a large category hierarchy,” AnnualInternational ACM-SIGIR Conference on Research and Development inInformation Retrieval, US, New York, N.Y.: ACM, 1997, pages 246-255;EP-A-0 694 829 (XEROX Corp.); U.S. Pat. No. 5,644,740 (Kiuchi Itsuko);Gert Schmeltz Pedersen: “A browser for bibliographic informationretrieval, based on an application of lattice theory,” Proceedings ofthe Annual International ACM SIGIR Conference on Research andDevelopment in Information Retrieval, US, New York, ACM, vol. CONF., 16,1993, pages 270-279; and Story G. et al: “The Rightpages image-basedelectronic library for alerting and browsing,” Computer, US, IEEEComputer Society, Long Beach, Calif., US, vol. 25, no. 9, 1 Sep. 1992,pages 17-25.

SUMMARY OF THE INVENTION

The present Applicants developed for such purpose a process solving theabove problems by an innovative use of taxonomies as a structuring andinformation access tool.

Dynamic taxonomies are a model to conceptually describe and access largeheterogeneous information bases composed of texts, data, images andother multimedia documents.

A dynamic taxonomy is basically a IS-A hierarchy of concepts, going fromthe most general (topmost) to the most specific. A concept may haveseveral fathers. This is a conceptual schema of the information base,i.e. the “intension”. Documents can be freely classified under differentconcepts at different level of abstraction (this is the “extension”). Aspecific document is generally classified under several concepts.

Dynamic taxonomies enforce the IS-A relationship by containment, i.e.the documents classified under a concept C are the deep extension of C,i.e. the recursive union of all the documents classified under C andunder each descendant C′ of C.

In a dynamic taxonomy, concepts can be composed through classicalboolean operations. In addition, any set S of documents in the universeof discourse U (defined as the set of all documents classified in thetaxonomy) can be represented by a reduced taxonomy. S may be synthesizedeither by boolean expressions on concepts or by any other retrievalmethod (e.g. “information retrieval”). The reduced taxonomy is derivedfrom the original taxonomy by pruning the concepts (nodes) under whichno document d in S is classified.

A new visual query/browsing approach is supported by dynamic taxonomies.The user is initially presented with the complete taxonomy. He/she canthen refine the result by selecting a subset of interest. Refinement isdone by selecting concepts in the taxonomy and combining them throughboolean operations. She/he will then be presented with a reducedtaxonomy for the selected set of documents, which can be iterativelyfurther refined.

The invention described here covers the following aspects of dynamictaxonomies:

1. additional operations;

2. abstract storage structures and operations on such structures for theintension and the extension;

3. physical storage structures, architecture and implementation ofoperations;

4. definition, use and implementation of virtual concepts;

5. definition, use and implementation of time-varying concepts;

6. binding a dynamic taxonomy to a database system;

7. using dynamic taxonomies to represent user profiles of interest andimplementation of user alert for new interesting documents based on suchprofiles of interest.

The above and other objects and advantages of the invention, as willappear from the following description, are obtained by a dynamictaxonomy process as claimed in claim 1. Preferred embodiments andnon-trivial variations of the present invention are claimed in thedependent Claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be better described by some preferredembodiments thereof, given as a non-limiting example, with reference tothe enclosed drawing, whose FIG. 1 shows a block diagram of the processof the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Before proceeding with a detailed description of the invention, suitableterminology remarks will be made. The set of documents classified underthe taxonomy (corpus) is denoted by U, the universe of discourse. Eachdocument d in U is uniquely identified by an abstract label calleddocument ID of d (DID(d)). Each concept c in the taxonomy is uniquelyidentified by an abstract label called concept ID of c (CID(c)).Concepts are partitioned into terminal concepts (concepts with noconcept son in the taxonomy) and non-terminal concepts. T denotes theset of concepts used in the taxonomy.

The taxonomy is usually a tree, but lattices (deriving from a concepthaving more than one father) are allowed. Documents can be classifiedunder any (terminal or non-terminal) concept in the taxonomy. A specificdocument d in U may be classified under one or more concepts. Thesingle, most general concept in the taxonomy is called the root of thetaxonomy. This concept need not be usually stored in the extension,since it represents the entire corpus.

The term “deep extension” of a concept c denotes all the documentsclassified under c or under any descendant of c. The term “shallowextension” of a concept c denotes all the documents directly classifiedunder c.

If c is a concept, C^(up)(c) denotes the set {c union {c′: c′ is anancestor of c in the taxonomy, and c is not the root of the taxonomy}}.C^(up)(c) is computed by the recursive application of operation AIO3(described hereinbelow). If c is a concept, C^(down)(c) denotes the set{c union {c′: c′ is a descendant of c in the taxonomy}}. C^(down)(c) iscomputed by the recursive application of operation AIO2 (describedhereinbelow).

With reference to FIG. 1, a block diagram is shown of the main steps ofthe process of the present invention, from which all furtherdevelopments of the process itself originate, such developments beingdescribed hereinbelow.

According to the diagram in FIG. 1, the process for retrievinginformation on large heterogeneous data bases of the present inventioncomprises the steps of:

(F1) initially showing a complete taxonomy for retrieval;

(F2) refining the retrieval through a selection of subsets of interest,where the refining step is performed by selecting concepts in thetaxonomy and combining them through boolean operations;

(F3) showing a reduced taxonomy for the selected set; and

(F4) further refining the retrieval through an iterative execution ofthe refining and showing steps.

In addition to the previously-described operations, the followingoperations can be supported:

-   a. projection under a given CID of a set S of DIDs: it extracts all    the children c of CID such as there is at least a document in S in    the deep extension of c-   b. extracting the CID's for a specific document d in U.

The prior art has never specified storage structures nor theimplementation of operations, that are both presented in this context.Abstract storage structures are defined with the following notation.Given domains A1, . . . , AN and B1, . . . , BM:

-   -   the relation R:[A1, . . . , AN]→[B1, . . . , BM] means that a        N-uple of values drawn from domains A1, . . . , AN uniquely        identifies an M-uple of values drawn from domains B1, . . . ,        BM. If [A1, . . . , AN]→[B1, . . . , BM] holds, then any [A1, .        . . , AN]→[Bi] holds, where Bi is drawn from any domain in the        set {B1, . . . , BM}    -   the relation R: [A1, . . . , AN]→{B1, . . . , BM} means that a        N-uple of values drawn from domains A1, . . . , AN uniquely        identifies a set of M-uples of values drawn from domains B1, . .        . , BM. If [A1, . . . , AN]→{B1, . . . , BM} holds, then any        [A1, . . . , AN]→{Bi} holds, where Bi is drawn from any domain        in the set {B1, . . . , BM}.

When brackets are omitted in the right part, square brackets areassumed.

Abstract relations can be trivially mapped (for the purpose ofillustration, and with no intent to restrict their representation) torelations in a relational schema, in the following way:

R: R: [A1, . . . , AN]→[B1, . . . , BM] maps into R(A1, . . . , AN, B1,. . . , BM)R: R: [A1, . . . , AN]→{B1, . . . , BM} maps into a set of 4^(th) NFrelations Ri(A1, . . . , AN, Bi)where underlined domains are key attributes of R.

Abstract SQL queries on these relations will be used to expressoperations. When expedient, the notation A.B applied to an abstractrelation [A]→[B] or [A]→{B} will be used to denote the value or the setof values of B corresponding to a given value of A. Domain CID holds theabstract labels of concepts, i.e. stands for the set of values {CID(c),for all c in the taxonomy}. Domain DID holds the abstract labels ofdocuments, i.e. denotes the set of values {DID(d), for all d in U}.

Abstract structures to store the intension will now be described.

The intension is the taxonomy itself; it can be seen as a conceptualschema for a set of corpora. The intension is stored as:

AIS1. One or more “dictionary” relations in the formDi: [CID]→[textualLabel]storing the user-visible definition of each concept; the domain“textualLabel” holds natural language descriptions of concepts. Eachdictionary can be in a different “language”, thereby allowingmultilingual corpora and/or different descriptions of concepts.AIS2. A language directory, identifying the appropriate dictionaryrelation for a specific “language” (required only if more than one“language” for concept description is used) in the form:

LD:[LANGUAGE_ID]→D

where LANGUAGE_ID holds the abstract identification of languages and Dholds the existing dictionaries.

An alternate representation of AIS1, AIS2 is by a single relation

AIS1′: [CID, LANGUAGE_ID]→textualLabel.AIS3. A father to son relation in the form

FS: [CID]→{SON_CID}

or

FS′: [CID, SEQ]→[SON_CID]

storing, for each concept c, its sons in the taxonomy. The domainSON_CID is the same as CID. The domain of SEQ is the set of naturalnumbers.

The second form, which is generally used, allows to supply a meaningfuldisplay order among the sons of a concept c.

AIS4. A son to father relation, in the form

SF: [CID]→{FATHER_CID}

storing, for each concept c, its fathers in the taxonomy. The domainFATHER_CID is the same as CID. If the taxonomy is not a lattice (i.e.any concept c can have no more than one father), this relation becomes:

SF: [CID]→[FATHER_CID]

In this latter case, information on the father of a specific concept cmay alternatively be stored in the dictionaries as:

Di: [CID]→FATHER_CID, textualLabelalthough this results in redundancy if more than one dictionary ismaintained.

Abstract storage structures for the extension will now be described.

The extension represents the classification of documents. As such, itdepends on the specific corpus. The extension is abstractly representedby the following three relations:

AES1. Deep Extension, in the Form DE:[CID]→{DID}

storing, for each concept c, all the documents in its deep extension(that is, all the documents classified under c or under any descendantc′ of c).

AES2. Shallow Extension, in the Form

SE: [CID]→{DID} equivalent to [CID, DID]storing, for each concept c, all the documents in its shallow extension(that is, all the documents directly classified under c). The shallowextension and the deep extension are the same for terminal concepts, sothat for such terminal concepts only one of DE and SE needs to be kept(typically, DE will be kept).

AES3. Classification, in the Form CL: [DID]→{CID}

storing, for each document, the most specific concepts under which it isclassified. All the ancestors of these concepts can be easily recoveredthrough the son-to-father (SF) relation in the intension. This structureis required only if the display of the classification for storeddocuments is supported at the user level. This storage structure isoptional, since the set K of concepts under which a specific DID isstored can be synthesized by operation AE05 applied to each concept c inT on the singleton set {DID}. A concept c is then in K if and only ifoperation AE05 returns TRUE.

AES4. Document Directory

Not specified, since it depends on the host system. It maps a documentid into information required to retrieve the specific document (forexample, the file name).

The abstract implementation of operations on the intension will now bedescribed.

AIO1. Given a concept c identified by K=CID(c), find its label in aspecific language L.1. Access the appropriate language directory

SELECT D FROM LD WHERE LANGUAGE_ID=L

2. Use K as a key to access the textual labelSELECT textualLabel

FROM D WHERE CID=K

AIO2. Given K=CID(c) find all its sons.Access the father-to-son relation FS, using K as a partial key

SELECT SON_CID FROM FS WHERE CID=K Or

Access the father-to-son relation FS′, using K as a partial key

SELECT SEQ, SON_CID FROM FS′ WHERE CID=K ORDER BY SEQ, SON_CID

AIO3. Given a K=CID(c), find all its fathers.Access the son-to-father relation SF, using K as a partial key

SELECT FATHER_CID FROM SF WHERE CID=K

AIO4. Insert, delete, change operations.Insert operations are performed by inserting the new concept C:

-   -   in the dictionaries (AIS1)    -   in the father to son relation (AIS3)    -   in the son to father relation (AIS4)

If C is a son of another concept C′, it may be useful to allow the userto reclassify under C some of the documents presently classified in theshallow extension of C′.

In the case in which each concept has a single father in the taxonomy,the deletion of a concept C is performed by deleting from the intension(AIS1, AIS3, AIS4) all concepts cεC^(down)(C). In addition (in order toavoid losing documents), the documents in the deep extension of C shouldbe added to the shallow extension of C′, where C′ is the father of C inthe taxonomy, unless C′ is the root of the taxonomy. The shallow (AES2)and deep (AES1) extensions for all concepts cεC^(down)(C) must beremoved. The concepts in C^(down)(C) must be removed from theclassification (AES3) of all the documents in the deep extension of C.

Alternatively, and in the general case in which concepts can havemultiple fathers, we proceed as follows.

Define LinkDelete(f, s) as:

-   1. remove from AIS3 the instance where CID=CID(f) and SON_CID=CID(s)-   2. remove from AIS4 the instance where CID=CID(s) and    FATHER_CID=CID(f)

Define BasicDelete(c) as:

-   1. for each f in {f: f is a father of c} call LinkDelete(f, c)-   2. remove the deep (AES1) and shallow (AES2) extension for c, its    classification (AES3), and any dictionary entries associated with c.

Define RecursiveDelete(f, s) as:

-   1. if f is the only father of s then    -   1.1. for each s′ in {s′: s′ is a son of s} call        RecursiveDelete(s, s′)    -   1.2. call BasicDelete(s)-   2. else call LinkDelete(f, s)

Define RecomputeDeepExtension(c) as:

-   1. for each s in {s: s is a son of c}    -   1.1. set the deep extension of c:        DeepExtension(c)=DeepExtension(c) union        RecomputeDeepExtension(s)-   2. return(DeepExtension(c))

Define UpdateDeepExtension(c) as:

-   1. for each f in {f: f is a father of c}    -   1.1. DeepExtension(f)=DeepExtension(c) union ShallowExtension(f)    -   1.2. UpdateDeepExtension(f)        Deletion of c is then implemented as:-   1. Compute the set F(C), which represents all the fathers of the    concept to be deleted (accessible through relation AIS4). All and    only the concepts in F(C) and their ancestors will have their deep    extension affected by the deletion of C.-   2. For each s in {s: s is a son of C}, call RecursiveDelete(C, s)-   3. Call BasicDelete(C).-   4. Recompute the deep extension of all the fathers of C: for each f    in F(C) call RecomputeDeepExtension(f)-   5. Update the deep extension of all the ancestors of the set F(C):    -   5.1. For each f in F(C) call UpdateDeepExtension(f)

Changes in the taxonomy may be of three types:

-   1. changing the labeling of a concept C: this only requires the    modification of the textualLabel in AIS1-   2. changing the place of a concept C in the taxonomy-   3. adding an additional father C′ to C in the taxonomy

In case 2, let C′ be the current father of C and C″ the new father of C.First, C must be deleted from the taxonomy, and reinserted with C″ as afather. The deep extension of C must be deleted from the deep extensionof all concepts cεC^(up)(C′) (by set subtraction, or by applying theabove algorithm for deletion with steps 2 and 3 replaced by Creparenting). The deep extension of C must be added to the deepextension of all concepts cεC^(up)(C″) (by set union). No changes inshallow extensions are required.

In case 3, the deep extension of C must be added to the deep extensionof all concepts cεC^(up)(C′) (by set union).

The abstract implementation of operations on the extension will now bedescribed.

AEO1. Given a concept c such that CID(c)=K, find its deep extension.Access the deep-extension relation DE, using K as a partial key

SELECT DID FROM DE WHERE CID=K

AEO2. Given a concept c such that CID(c)=K, find its shallow extension.Access the shallow extension relation SE, using K as a partial key

SELECT DID FROM SE WHERE CID=K

AEO3. Test the membership of a set of DIDs {DID} in the deep extensionof a concept CID.

-   1. Retrieve the deep extension of CID-   2. For each d in {DID}, test whether d belongs to the    deep-extension; if it does, return TRUE; if no d in {DID} does,    return FALSE    AEO4. Given a set of DIDs {DID}, count the number of documents in    {DID} which are also in the deep extension of CID.-   1. Retrieve the deep extension of CID-   2. Initialize CNT to 0-   3. For each d in {DID}, test whether d belongs to the    deep-extension; if it does, CNT=CNT+1-   4. Return CNT    AEO5. Test the membership of a set of DIDs {DID} in the shallow    extension of a concept CID.    As in AEO3, by substituting the deep extension with the shallow    extension.    AEO6. Given a set of DIDs {DID}, produce the projection under a    concept CID.-   1. Retrieve the set {SON} of all the sons of CID-   2. Initialize set R to empty-   3. For each concept s in SON, use operation AEO3, or operation AEO4    if counters are desired, to test the membership of {DID} in s. If    the operation returns TRUE (>0 if AEO4 is used) add to list R-   4. Return R    AEO7. Given a set of DIDs {DID}, produce the reduced taxonomy for    {DID}.

As a clarification, the set of DIDs for which the reduced taxonomy hasto be produced can be generated by operations on the taxonomy and alsoby any other means, including, without loss of generality, databasequeries and information retrieval queries. Also, the current combinationof concepts can be used as a pre-filter for other retrieval methods.

For performance reason, the reduced taxonomy is usually produced ondemand: the request only displays the highest levels in the tree. Theset {DID} is kept in memory, so that when the explosion of a specificconcept in the reduced taxonomy is requested, appropriate filtering isperformed.

1. Produce the projection of {DID} for the root On the subsequentexplosion of concept c:

Produce the projection of {DID} for c:

The reduced tree can also be totally computed in a single step. Let RTbe the set of concepts in the reduced tree. RT can be computed bytesting, for each concept c in T, the membership of {DID} in c throughoperation AEO3 or AEO4 (if counters are required). Concept c is in RT ifand only if operation AEO3 returns TRUE or operation AEO4 returns acounter larger than 0.

The computation can be speeded up in the following way:

-   1. Initialize a table S of size |T|, where S[i] holds information on    the current status of concept i, initialized at “pending”.-   2. Starting from the uppermost levels, and continuing down in the    tree, process concept i.    -   2.1. If S[i] is “empty”, i does not belong to RT, and processing        can continue with the next concept.    -   2.2. If S[i] is not “empty”, apply operation AEO3 or AEO4 to i.        -   2.2.1. If the operation returns TRUE (AEO3) or a counter            larger than 0 (AEO4), i belongs to RT.        -   2.2.2. Otherwise, neither i nor any of its descendants            belong to RT: set to “empty” all S[j] in S, such that j is a            descendant of i in the taxonomy. Descendants can be            efficiently obtained by keeping a precomputed table D,            holding for each concept in the taxonomy a list of all the            concepts descending from it in the taxonomy (such a table            must be recomputed every time the taxonomy changes).            AEO8. Boolean combination of concepts.

Boolean combinations of concepts are performed through the correspondingset operations on the deep extension of concepts. Let c and c′ be twoconcepts, and DE(c) and DE(c′) their deep extension (represented byAES1):

c AND c′ corresponds to DE(C)∩DE(c′)c OR c′ corresponds to DE(c)∪DE(c′)c MINUS c′ corresponds to DE(c)−DE(c′)NOT c corresponds to U-DE(c), where U is the universeAEO9. Insertion of a new document.The insertion of a new document d (represented by DID(d)) classifiedunder a set of concepts {C} requires the following steps: for each cε{C}

-   1. insert DID(d) in the shallow extension of c (AES2), if c is not a    terminal concept and the shallow extension must be stored-   2. insert DID(d) in the deep extension (AES1) of C^(up)(c).-   3. insert an item [DID(d)]→{C} in the classification structure AES3    AEO10. Deletion of an existing document.    The deletion of a document d (represented by DID(d)) requires the    following steps:-   1. retrieve the set of concepts {C} under which d is shallowly    classified, by accessing AES3 with DID(d) as the key (operation    AEO2)-   2. for each cε{C}    -   a. delete DID(d) from the shallow extension of c    -   b. for all c′εC^(up)(c): delete DID(d) from the deep extension        of c′-   3. delete the entry corresponding to DID(d) from AES3.

If AES3 is not stored, deletion is performed in the following way. Foreach concept c in T, if d belongs to the shallow extension of c:

-   1. delete DID(d) from the shallow extension of c-   2. for all c′εC^(up)(c): delete DID(d) from the deep extension of c′    AEO11. Document reclassification.

Changes in the classification of a document d (represented by DID(d))are implemented in the following way. Let d be initially classifiedunder a concept c (possibly null) and let the new concept under which dmust be classified be c′ (possibly null). If both c and c′ are non-null,the operation means that d was previously classified under c and mustnow be classified under c′; if c is null, the operation means that d isadditionally classified under c′; if c′ is null, the operation meansthat the original classification under c must be removed. At least oneof c and c′ must be non-null. If c is not null:

-   1. eliminate DID(d) from the shallow extension (AES2) of c-   2. eliminate DID(d) from the deep extension (AES1) of all    c″εC^(up)(c)-   3. eliminate c from the classification of d (AES3)    If c′ is not null:-   1. insert DID(d) in the shallow extension (AES2) of c′ (if the    shallow extension of c exists)-   2. insert DID(d) in the deep extension (AES1) of all c″εC^(up)(c′)-   3. insert c′ in the classification of d (AES3)    AEO12. Find the concepts under which a document d is immediately    classified.    Retrieve {C} from AES3, using DID(d) as a key.

Physical storage structures, architecture and implementation ofoperations will now be described.

As regards the intension, storage structures usually contribute with anegligible overhead to the overall storage cost, since a few thousand ofconcepts are usually adequate even for semantically rich corpora.Storage for these structures may be provided by any database managementsystem or any keyed access method. The second form of AIS3 (FS′)requires an ordered access, since SEQ is used to order the sons of aspecific concept. Because of the low overhead, all the intensionalstorage structures (with the possible exception of AIS1, thedictionaries) may be usually kept in central memory.

As regards the extension, the most critical component is AES1 (the deepextension), for several reasons. First, deep-extension semantics are thenatural semantics for boolean combinations of concepts (see AEO8).Second, the production of reduced taxonomies requires a possibly largenumber of projections (which are performed on the deep extension), whoseperformance is critical for visual operations.

It is critical that the deep extension of concept c is explicitlystored, and not computed as the union of the shallow extensions of allthe descendants of c.

Although any dbms or keyed access method can be used to provide storagefor the deep extension, the set of documents in the deep extension canbe more efficiently represented than by straightforwardly mapping theabstract relation.

The use of fixed size bit vectors in the present context will now bedescribed. Information data bases with a small-to-moderate number ofdocuments can effectively represent the deep extension of a concept c bybit vectors, each of size equal to |U′|, the maximum number of documentsin the universe. In the bit vector, bit i is set if and only if thedocument d with DID(d)=i is in the deep extension of c.

Set operations on the deep extension only involve logical operations onbit vectors (AND, OR, NOT, etc.). These operations take one or more bitvectors and produce a result bit vector of the same size.

Let document id's be numbered 0 to |U′|−1, and n be the number of bitsin the word of the host CPU. For performance reasons, it is better toset the fixed size of bit vectors at ┌|U′|/n┐, in order to be able toperform bit operations at the word level. Unused bit positions are leftunset.

Counting the number of documents in the result of any operation can beefficiently performed by table lookup, in the following way.

Let the unit of access UA (not necessarily the CPU word) be n bits.Build once a vector V of 2^(n) elements, stored in memory, which storesin V[i], the number of bits set in the binary number 2^(i),0<=i<=2^(n)−1.

Counting:

Initialize counter C at 0;Access the bit vector in chunks of n bits at a time:for each chunk

store the chunk in i

set C=C+V[i]

For access at the octet level (n=8), the translation table requires nomore than 256 octets. For access at the double octet level (n=16), nomore than 64K octets. Larger units of access are not recommended.

Insertion, deletion and reclassification are also efficiently performed,by simply locating the appropriate deep and/or shallow extension andsetting/resetting the appropriate bit.

This same representation can be trivially used for storing structuresAS2 and AS3. In AS3 the size of the bit vector is equal to thecardinality of the set of concepts in the taxonomy.

As regards compressed bit vectors, by construction, the deep extensionis very sparse at terminal level, and very dense at the top levels inthe taxonomy. The use of any type of bit vector compression (such as,without prejudice to generality, Run Length Encoding (see Capon J., “Aprobabilistic model for run-length coding of pictures”, IEEE Trans. onInf. Theory, 1959) and/or variable-length bit vectors) is thereforebeneficial in reducing the overall storage overhead, although itintroduces a compression/decompression overhead.

If a controlled error-rate in operations is acceptable, Bloom filters(see Bloom, B. H., Space/time tradeoffs in hash coding with allowableerrors, Comm. of the ACM, 1970) can be used to represent the deepextension in a compact form, suitable for larger information bases. WithBloom filters, counting and set negation are usually not supported.

For large to very large information bases, a bit vector representation(albeit compressed) may produce an excessive storage overhead. The deepand shallow extensions as well as structure AES3 may be stored asinverted lists (see Wiederhold, G., Files structures, McGraw-Hill,1987). Because of performance in the computation of set operations, suchlists (and the result of set operations) are kept ordered by documentid's. For the above-cited statements, it is generally advantageous touse any form of inverted list compression.

As regards the general architectural strategies, the implementation ofdynamic taxonomies should try to keep all the relevant data structuresin main memory, shared by the processes accessing them.

As noted before, the intension overhead is generally negligible so thatintensional structures (with the possible exception of dictionaries) maybe usually kept in memory without problems.

Extension overhead for extensional structures is considerably larger. Ifthe storage overhead prevents the complete storage of deep-extensionstructures, buffering strategies should be used, such as LRU or the onesdescribed in documents Johnson, T., Shasha D.: 2Q: A Low Overhead HighPerformance Buffer Management Replacement Algorithm, Int. Conf. on VeryLarge Databases, 1994; and O'Neill, et al.: The LRU-K Page ReplacementAlgorithm For Database Disk Buffering, SIGMOD Conf. 1993. Shallowextensions and classification structures are less critical and may bekept on disk (again with the buffering strategies described in the twoabove-mentioned documents).

As indicated in operation AEO3, the membership test without counting canreturn TRUE when the first DID common to both lists is found, therebyspeeding up the computation.

The use and implementation of virtual concepts will now be described.

Some data domains (such as price, dates, quantities, etc.) correspondusually to a concept (e.g. PRICE) which can be expanded into a largenumber of terminal concepts, each representing a specific value (e.g.100$). Such a representation causes a high number of son concepts, andincreases the complexity of the taxonomy. Alternatively, values can begrouped by defining meaningful intervals of values and representing onlythe intervals as specific concepts. This representation loses the actualdata, and presents the user with a fixed classification. Grouping mayalso be combined with exhaustive representation, but inherits most ofthe problems of both schemes.

The invention of “virtual concepts” provides a third, more flexiblealternative. We define a “Simple virtual concept” as a concept for whichneither the actual sons (actual values of the domain to be represented)nor the actual extension are stored, but are computed (usually fromadditional, possibly external data).

A virtual concept is completely described by 4 abstract operations:

V1: Given a virtual concept v, retrieve all its sons.V2: Given a virtual concept v, retrieve its deep extension.V3: Given the son s of a virtual concept v, retrieve its deep extension.V4: Given a document d, find all the terminal concepts (descendants ofv) under which it is stored.

One way of implementing these abstract operations is by keeping, foreach virtual concept

v, two abstract relations:S_(v): [value]→{DID}which stores the set of documents with a given value in the domain ofvalues of the virtual concept.

C_(v): [DID]→{value}

which stores the set of values for a specific document; if each documenthas a single value C_(v): [DID]→[value]. A single C_(v) relation maystore multiple domains and be shared by many virtual concepts: in thiscase C_(v): [DID]+{valueA, . . . , valueN}, where valueI denotes the setof values for domain I. It is important to note that neither S_(v) norC_(v) need to be explicitly stored, but they can be also synthesized byqueries on external data.

These two abstract relations can be represented by a single relation ina relational schema (without loss of generality and simply to provide aclear description of operations)

C_(v)(DID, value)with underscored attributes representing the primary keys. S_(v)actually stores the inversion of C_(v) and will usually be representedby a secondary index on C_(v), rather than by a base relation.

With this representation, the abstract operations defined before can beeasily implemented by SQL queries:

V1: Given a virtual concept v, retrieve all its sons:SELECT DISTINCT value

FROM C_(v)

V2: Given a virtual concept v, retrieve its deep extension:

Select Distinct Did FROM C_(v)

V3: Given the son s of a virtual concept v, retrieve its extension (s isa terminal concept, so that its deep and shallow extension are the same)

SELECT DISTINCT DID FROM C_(v)

WHERE value=sCounting is trivially added.V4: Given a document d, find all the terminal concepts (descendants ofv) under which it is storedRETRIEVE DISTINCT value

FROM Cv WHERE DID=d

In general, a virtual concept v can be organized into a sub-taxonomy,i.e. each non-terminal son of v represents a set of actual domainvalues. Each son may be further specialized, and so on. For instanceSALARY can be organized into the following taxonomy:

SALARY Low (e.g. <1000) Medium (e.g. >=1000 and <10000) High (e.g.>10000)

In this case, the non-terminal descendants of v can be stored as derivedvirtual concepts, i.e. virtual concepts referencing the same abstractrelations defined for v, but providing additional restrictions. In theexample, “Low” can be characterized by the additional restrictionvalue<1000, so that operation V3 for Low becomes:

SELECT DISTINCT DID FROM C_(v)

WHERE value<1000

Virtual and derived virtual concepts are peculiar in that their terminaldescendants and their extensions are not directly stored but computed.In order to represent them in our framework, the following abstractrelations are added to the intension:

AIS5: [CID]→[conceptType]where conceptType designated real, simple virtual and derived virtualconcepts.

AIS6: [CID]→[S_(CID)]

for simple virtual concepts, stores the abstract relation Sv (which cansynthesized be a query) for the virtual concept CID

AIS7: [CID]→[CCID]

for simple virtual concepts, stores the abstract relation Cv (which cansynthesized be a query) for the virtual concept CIDAIS8: [CID]→[CID′, restriction]for derived virtual concepts only, identifies the virtual concept torefer to and the additional restriction.

The use and implementation of time-varying concepts will now bedescribed.

Time-varying concepts, such as age, can be represented by a simplevariant of virtual concepts. A time instant t is represented as anabstract “timestamp”. The timestamp contains the number of clock ticksstarting from a fixed time origin; the clock resolution depends on theapplication. All timestamps use the same time coordinates. Thedifference between two timestamps t and t′ defines the time intervalamplitude between the two times. Let the values of the virtual concept vbe the set of timestamps of all documents in the extension of v, and letT be the timestamp of the current time, and the sons of v be representedas time intervals with respect to the current timestamp T: Given avirtual concept v, retrieve all its sons:

SELECT DISTINCT T-value FROM C_(v)

Given a virtual concept v, retrieve its deep extension:

SELECT DISTINCT DID FROM C_(v)

Given the son s of a virtual concept v, retrieve its extension

SELECT DISTINCT DID FROM C_(v)

WHERE value=T−s

Alternatively, and more efficiently, the values of the time-varyingconcept can be split into N intervals (from more recent to older), whichare stored as real concepts. In addition, for each interval I, we keep:

-   a. the list L(I) of DIDs in the interval ordered by decreasing    timestamps (i.e. newer to older)-   b. in central memory, an interval representative IR(I): the last DID    in the interval together with its timestamp-   c. a classification criterion (e.g. T-value less than 1 week and no    smaller than 1 day)

Since the classification of documents varies with time, we need tore-compute the classification of documents every time tick (arbitrarytime interval selected by the system administrator, typically a multipleof clock resolution), according to the following algorithm:

At each time tick: For each interval I while IR(I) needsreclassification (i.e. it fails the classification criterion for I) do {  Reclassify(IR(I));   set as IR(I) the last DID in the ordered list a)} where Reclassify(IR(I)) is Delete IR(I).DID from I For(i=i+1 to N) {  if IR(I).timestamp meets the classification criterion for interval i  {     insert IR(I) in interval i     break;   } }

Binding a dynamic taxonomy to a database system will now be described.

The present invention allows to use a dynamic taxonomy to browse andretrieve data stored in a conventional dbms (relational,object-relational, object-oriented, etc.). The invention covers datastored as a single relation (or object) or, more generally, representedby a single view on the database (see Elmasri, Navathe, Fundamentals ofdatabase systems, The Benjamin/Cummings Publ. Co., 1994).

In this case documents correspond to tuples (or rows, records, objects)in the view V. In order to identify a document we can either use theprimary key of the view as a document identifier (DID) or keep twoabstract relations mapping system-generated DID's to and from theprimary key PK of the view:

DK: [DID]→[PK] IDK: [PK]→[DID]

where PK represents the primary key of the relation. DK is used toaccess a tuple of V, given a document id DID, and IDK is used toretrieve the document id corresponding to a specific value in theprimary key of V. This latter representation is beneficial when primarykeys PK's are large (e.g. when they are defined on alphanumericattributes).

Given a view V we can construct a taxonomy T for V in the following way.For each attribute A in V, we place a corresponding concept C(A) (eithera real or a virtual one) as an immediate son of the root. Virtualconcepts use V itself for the synthesis of sons and extensions (aspreviously seen). Real concepts can be further specialized as requiredby the semantics of A.

Given a tuple t in V, for each attribute A in V, let t.A denote thevalue of attribute A in t. For each real concept C in T (either C(A) ora descendant of C(A)), the designer must provide a boolean clause B(C,t) such that t (represented by DID(t)) is to be classified under C ifand only if B(C, t)=TRUE.

The boolean clause B(C, t) may reference any attribute of t, andconsequently, new virtual concepts (called “extended concepts”) may bedefined on combinations of attributes by operations on the database(including but not restricted to sums, averages, etc. of databasevalues).

A special case occurs when the boolean clause B(C, t) is true whent.AεS_(C), where S_(C) is a set of values of attribute A andS_(C)∩S_(C′)=Ø, for ∀C≠C′. In this case, it is more efficient to keep atable T:[v]→[c], listing for each value v in domain(A), thecorresponding concept c. If S_(C)∩S_(C′)≠Ø, for ∃C≠C′, multiple conceptscan be associated with the same value, so that T:[v]→{c}.

In addition to this mapping among attributes and concepts, the designermay define new concepts either as taxonomic generalizations ofattributes or extended concepts.

-   New taxonomic generalizations. For virtual concepts, this feature    was discussed previously. If the sons of a new taxonomic    generalization G are real concepts {S}, no boolean clause is usually    required for G, because classification under G is automatically    performed by operation AEO9.-   Extended concepts. New concepts may be derived either as real or    virtual concepts by operations on the database (including but not    restricted to sums, averages, etc. of database values).

Binding is then performed in the following way. Virtual concepts do notrequire any special processing, since they are realized by operations onthe database. Real concepts require a classification for any new tuple,a deletion if t is deleted or a reclassification if t is changed. Inorder to classify t, the system locates the set C of concepts for whichB(c, t), cεC is satisfied and classifies t under ∀cεC (and, consequentlyunder all of c's ancestors). Deletion and reclassification are performedas previously stated.

Example:

Given the relation R:(TOWNID, NAME, COUNTRY, POPULATION), we canidentify the documents in the database by the values of TOWNID. We needto decide which attributes will be represented in T and how they will berepresented. Let COUNTRY be represented by a real concept, and NAME berepresented by a virtual concept. In addition we define the real conceptCONTINENT as the continent the COUNTRY is in. CONTINENT can berepresented in two ways: as a taxonomic generalization concept or as anextended concept.

If we represent CONTINENT as an extended concept, the taxonomy T willbe:

NAME   Sv:Select TOWNID FROM R WHERE NAME = x   Cv:Select DISTINCT NAMEFROM R CONTINENT EUROPE t.COUNTRY=“Italy” or t.COUNTRY=“France” or ...AMERICA t.COUNTRY=“USA” or ... ASIA t.COUNTRY=... COUNTRY   Italyt.COUNTRY=“Italy”   France t.COUNTRY=“France”   Usa t.COUNTRY=“USA”  ...

If we represent CONTINENT as a taxonomic generalization of COUNTRY, thetaxonomy T′ will be:

NAME   Sv:Select TOWNID FROM R WHERE NAME = x   Cv:Select DISTINCT NAMEFROM R CONTINENT   EUROPE    Italy t.COUNTRY=“Italy”    Francet.COUNTRY=“France”   AMERICA    Usa ...    ...   ASIA    ... COUNTRY  Italy t.COUNTRY=“Italy”   France t.COUNTRY=“France”   Usat.COUNTRY=“USA”   ...

In both cases, NAME is represented in the same way. For NAME, we havetwo abstract relations

Sv: [COUNTRY]→{TOWNID} Cv: [TOWNID]→[COUNTRY]

POPULATION is represented in an analogous way.

Finally, the use of dynamic taxonomies to represent user profiles ofinterest and implementation of a user alert for new interestingdocuments based on dynamic taxonomy profiles, will be described.

The invention consists in using set-theoretic expressions on concepts(plus optional, additional expressions, such as information retrievalqueries) to describe user interest in specific topics. Such expressionsmay be directly entered by the user or transparently and automaticallycaptured by the system, by monitoring user query/browsing. Thespecification of user profiles is especially important in electroniccommerce and information brokering and in monitoring dynamic datasources in order to advise users of new or changed relevant information.The information base is assumed to be classified through dynamictaxonomies.

The scenario is as follows. Several users express their intereststhrough possible multiple conceptual expressions, called “interestspecifications”. A monitoring system accepts these requests (with anabstract user “address” to send alerts to). The monitoring system alsomonitors an information base for changes (insertion, deletion, change).The information base is described by the same taxonomy used by users toexpress their interests.

When a change occurs in the information base (the type of change to bealerted for may be specified by users), the system must find the usersto alert on the basis of their interests.

A brute force approach will check all user interest specificationsexhaustively, and compute whether each changed document d satisfies anygiven specification S. We can test whether a document d satisfies aspecification S by applying the query specified in S to the singletonset {d} and test if d is retrieved. However, this strategy requires toperform, for each information base change, as many queries as there areuser specifications and may be quite expensive in practice. For thisreason, we define alternate strategies which reduce the number ofevaluations required.

We are primarily interested into the efficient solution of dynamictaxonomy specifications. Additional expressions, such as informationretrieval queries, will usually be composed by AND with taxonomicexpressions, and can therefore be solved, if required, after thecorresponding taxonomic expression is satisfied.

We will start from the simplest case, in which:

-   a) the specification is expressed as a conjunction of terminal    concepts;-   b) documents are classified under terminal concepts only.

As regards conjunctive specifications and document classification underterminal concepts only, we use two abstract storage structures:

1. a directory of specifications, in the form:

SD: [SID]→[N, SPEC]

where SID is an abstract identifier which uniquely identifies thespecification, SPEC is the specification itself (optional), N is thenumber of concepts referenced in the specification. Optionally, otherfields (such as the user “address”) will be stored in this structure.2. a specification “inversion”, in the form:

SI: [CID]→{SID}

listing for each concept c (represented by its concept identifier) allthe specifications (represented by their specification id) using thatconcept.

When a specification is created, its abstract identifier is created, itsdirectory entry is created in SD and the set of concepts referenced inthe specification are stored in the inversion SI.

When a document d is inserted, deleted or changed, let C be the set ofconcepts (terminal concepts by assumption) under which d is classified.The set of specifications that apply to d are then found in thefollowing way.

Let K be the set of concepts used to classify document d. For eachconcept k in K, let SID(k) be the list of specifications for k(accessible through relation SI) ordered by increasing specificationid's. We define MergeCount(K) as the set composed of pairs (SID, N) suchthat SID is in MergeCount(K) if SID belongs to a SID(k), k in K. If thepair (SID, N) is in MergeCount(K), N counts the number of SID(k)referencing SID. MergeCount(K) can be produced at a linear cost, bymerging the SID(k) lists.

Let S be a set initially empty, which represents the set ofspecifications satisfied by d.

For each pair (SID, N)

retrieve SID.N from SD;

if SID.N=N: S=S union SID

As regards specifications using unrestricted set operations, let S(represented by SID(S)) be a specification. Transform S into adisjunctive normal form (i.e. as a disjunction of conjunctions). Leteach conjunctive clause in S be called a component of S. We denote bySIDi(S) the i-th component of S.

Store the directory of specifications as two abstract relations:

SD (as before, with N omitted)SCD: [COMPONENT]→[SDI, N], where COMPONENT stores components ofspecifications, COMPONENT.SDI represents the specification id of thespecification S of which COMPONENT is a component, and COMPONENT.N isthe number of concepts referenced in the component.

The specification inversion is stored as:

SI: [CID]→{COMPONENT}, where CID is a concept identifier andCID.COMPONENT is the set of components referencing the conceptidentified by CID.

Let K be the set of concepts used to classify document d, for eachconcept k in K, let COMPONENT(k) be the list of components for k(accessible through relation SI) ordered by increasing component id's.Define ComponentMergeCount(K) as the set composed of pairs (COMPONENT,N) such that COMPONENT is in ComponentMergeCount(K) if COMPONENT belongsto a COMPONENT(k), k in K. If the pair (COMPONENT, N) is inComponentMergeCount(K), N counts the number of COMPONENT(k) referencingCOMPONENT. ComponentMergeCount(K) can be produced at a linear cost, bymerging the COMPONENT(k) lists.

Let S be a set initially empty.

For each pair (COMPONENT, N),retrieve COMPONENT.N through relation SCD;if COMPONENT.N=N: S=S union COMPONENT.SID (COMPONENT.SID is accessedthrough relation SCD).S represents the set of specifications satisfied by d.

As regards specifications and document classification under non-terminalconcepts to which they refer, the specification inversion SI needs to bemodified in the following way.

If a specification or component Z references concept C, represented byCID(C) then:

C is a terminal concept:

-   -   CID(C).SID=CID(C).SID union Z, if Z is a specification    -   CID(C).COMPONENT=CID(C).COMPONENT union

Z, if Z is a component

C is a non-terminal concept:

for each k in C^(down)(C)

-   -   CID(k).SID=CID(k).SID union Z, if Z is a specification    -   CID(k) COMPONENT=CID(k) COMPONENT union Z, if Z is a component

The set S of satisfied specifications is computed as per the previouscases.

The above-disclosed techniques allow computing the specificationssatisfied by a document d. In case it is desired to determine thespecifications satisfied by a set of documents D (whose cardinality isgreater than 1), the above-disclosed techniques can be applied in twoways. In the first way, the techniques are applied without modificationsto every document d in D, then removing possible duplicatespecifications. In the second way, K is defined as the set of conceptsused to classify D, the adequate technique is chosen among the describedones and the set S of “candidate” specifications is determined. Everyspecification s in S is then checked, performing it on D.

1. A method for retrieving person records for applications such asmatchmaking or human resource management, wherein retrieval is performedthrough visual queries on dynamic taxonomies, said dynamic taxonomiesbeing a hierarchical organization of concepts, said concepts alsocomprising features such as age of person and location of person, saidperson records being able to be classified under different concepts,said person records and their classification being called an extension,said method comprising: displaying a taxonomy for said retrieval;selecting a subset of interest of said taxonomy in order to refine saidretrieval, said subset of interest being specified by selecting taxonomyconcepts and combining said taxonomy concepts through booleanoperations, or being specified through querying methods, said queryingmethods retrieving classified person records according to differentselection criteria; displaying a reduced taxonomy for said selectedsubset of interest, said reduced taxonomy being derived from saidtaxonomy by eliminating from the extension of said taxonomy all personrecords not in said selected subset of interest and pruning conceptsunder which no person record in said selected subset of interest isclassified; and iteratively repeating said steps of selecting a subsetand of displaying a reduced taxonomy to further refine said retrieval,wherein: said hierarchical organization of concepts comprises a set offeatures, each of said features being a descendant concept of the rootconcept of said organization, each of said features having asdescendants in the taxonomy a set of concepts, each concept in said setof concepts representing either a single value or a set of values forsaid feature; said person records are classified, for each said feature,under zero or more concepts representing either a single value or a setof values for that feature; said step of displaying a reduced taxonomyeither reports only the concepts belonging to the reduced taxonomy or,for each such concept also reports how many person records in theinterest set are classified under the concept; and said step of pruningof concepts includes eliminating from the taxonomy the concepts underwhich no person record in the selected subset of interest is classified,or preventing such concepts from being selected in order to specifyinterest sets.
 2. A method for retrieving items for diagnosticapplications such as medical diagnosis or malfunction diagnosis, whereinretrieval is performed through visual queries on dynamic taxonomies,said dynamic taxonomies being a hierarchical organization of concepts,said concepts also comprising features such as symptoms, said itemsbeing able to be classified under different concepts, said items andtheir classification being called an extension, said method comprising:displaying a taxonomy for said retrieval; selecting a subset of interestof said taxonomy in order to refine said retrieval, said subset ofinterest being specified by selecting taxonomy concepts and combiningsaid taxonomy concepts through boolean operations, or being specifiedthrough querying methods, said querying methods retrieving classifieditems according to different selection criteria; displaying a reducedtaxonomy for said selected subset of interest, said reduced taxonomybeing derived from said taxonomy by eliminating from the extension ofsaid taxonomy all items not in said selected subset of interest andpruning concepts under which no item in said selected subset of interestis classified; and iteratively repeating said steps of selecting asubset and of displaying a reduced taxonomy to further refine saidretrieval, wherein: said hierarchical organization of concepts comprisesa set of features, each of said features being a descendant concept ofthe root concept of said organization, each of said features having asdescendants in the taxonomy a set of concepts, each concept in said setof concepts representing either a single value or a set of values forsaid feature; said items are classified, for each said feature, underzero or more concepts representing either a single value or a set ofvalues for that feature; said step of displaying a reduced taxonomyeither reports only the concepts belonging to the reduced taxonomy or,for each such concept also reports how many items in the interest setare classified under the concept; and said step of pruning of conceptsincludes eliminating from the taxonomy the concepts under which no itemin the selected subset of interest is classified, or preventing suchconcepts from being selected in order to specify interest sets.
 3. Amethod for the statistical comparison of different subsets of aninformation base, said information base being described by a dynamictaxonomy, said method comprising: initially displaying a view for eachof said subsets, said view being a reduced taxonomy derived from theinitial taxonomy by setting a specific focus; displaying for eachconcept in each view, a measure of statistical deviation from uniformityfor the subset represented by said concept with respect to the sameconcept in the first view only or in each of the other views includingthe first view, said measure of deviation only being a raw measure ofdeviation or including additional measures such as the statisticalsignificance of such deviation, said first view being used as areference view; selecting a subset of interest in any of said views,such subset of interest being automatically added to the subset ofinterest of each of the other views; and repeating said steps ofselecting subsets of interest and showing views, in order to comparedifferent subsets.
 4. A method for retrieving information on largeheterogeneous databases, wherein information retrieval is performedthrough visual queries on dynamic taxonomies, said dynamic taxonomiesbeing an organization of concepts that ranges from a most generalconcept to a most specific concept, said concepts and theirgeneralization or specialization relationships being an intension, itemsin said databases being able to be classified under different concepts,said items and their classification being called an extension, saidmethod comprising; displaying a taxonomy for said retrieval; selecting asubset of interest of said taxonomy in order to refine said retrieval,said subset of interest being specified by selecting taxonomy conceptsand combining the taxonomy concepts through boolean operations or beingspecified through querying methods, said querying methods retrievingclassified items according to different selection criteria; displaying areduced taxonomy for said subset of interest, said reduced taxonomybeing derived from said taxonomy by eliminating from the extension ofsaid taxonomy all items not in said selected subset of interest and bypruning concepts under which no item in said selected subset of interestis classified; and iteratively repeating said steps of selecting asubset of interest and of displaying a reduced taxonomy to furtherrefine said retrieval, wherein: said step of pruning of conceptsincludes eliminating from the taxonomy all the concepts under which noitem in the selected subset of interest is classified, or preventingsaid concepts from being selected in order to specify interest sets;said step of displaying a reduced taxonomy either reports only theconcepts belonging to the reduced taxonomy or, for each such conceptalso reports how many items in the interest set are classified under theconcept; said intension is organized as a hierarchy of concepts or as adirected acyclic graph of concepts, thereby allowing a concept to havemultiple fathers; items in said classification are classified manually,programmatically, or automatically; and said method is able toreconstruct relationships among concepts based on the classification, arelationship between any two concepts existing if at least one item isclassified (1) under a first concept or any descendants of the firstconcept, and (2) under a second concept, or any descendants of thesecond concept.
 5. The method according to claim 4, said processaccounting for the popularity of items by representing a numericalmeasure of popularity through a branch in the taxonomy.
 6. The methodaccording to claim 5, wherein said measure of popularity for an item isderived from the number of predefined actions, such as access orpurchase, applied to said item, or is supplied by an external source,such as user or editorial reviews.
 7. The method according to claim 4,said method allowing the personalization of interaction by permittingthe user to predefine a focus and having the system use said focusinstead of the universe for all user interactions.
 8. The methodaccording to claim 4, said method allowing the restriction of access fora specific user to a predefined set of items, where different users maybe restricted to different sets of items, and for all the interactionsof said specific user said set of items is used instead of the universe.9. The method according to claim 4, said method allowing to track userinterests by keeping track of all the concepts selected by users inorder to define foci.
 10. The method according to claim 9, said methodkeeping track of all the concepts selected by users in order to definefoci and of all the concepts expanded.
 11. The method according to claim4, said method performing the computation of the reduced taxonomy for aset of items, said process working on a relation containing a pair ofattributes, one of said attributes representing item identifiers, theother one of said attributes representing concept identifiers, in such away that if there is in such relation a tuple with a specific itemidentifier and a specific concept identifier then the item identified bysaid specific item identifier is classified under the concept identifiedby said specific concept identifier, said method comprising retrievingall the tuples containing an item identifier belonging to said set ofitems; counting the number of tuples in said retrieved tuples for eachconcept identifier, and producing said reduced taxonomy, concepts insaid reduced taxonomy being all the concept identifiers with a countlarger that zero, and the ancestors of said concepts.
 12. The methodaccording to claim 11, said method performing the counting of the numberof tuples for each concept identifier by grouping said tuples by conceptidentifiers, and counting for each said concept identifier the number oftuples belonging to said group.
 13. The method according to claim 11,further comprising saving computed interest sets in order to avoidrecomputation.
 14. The method according to claim 11, said furthercomprising saving computed reduced taxonomies in order to avoidrecomputation.
 15. A method for retrieving real estate items, whereinretrieval is performed through visual queries on dynamic taxonomies,said dynamic taxonomies being a hierarchical organization of concepts,said concepts also comprising features such as price and location ofreal estate items, said real estate items being able to be classifiedunder different concepts, said real estate items and theirclassification being called an extension, said method comprising:displaying a taxonomy for said retrieval; selecting a subset of interestof said taxonomy in order to refine said retrieval, said subset ofinterest being specified by selecting taxonomy concepts and combiningsaid taxonomy concepts through boolean operations, or being specifiedthrough querying methods, said querying methods retrieving real estateitems classified according to different selection criteria; displaying areduced taxonomy for said selected subset of interest, said reducedtaxonomy being derived from said taxonomy by eliminating from theextension of said taxonomy all real estate items not in said selectedsubset of interest and pruning concepts under which no real estate itemsin said selected subset of interest is classified; and iterativelyrepeating said steps of selecting a subset and of displaying a reducedtaxonomy to further refine said retrieval, wherein: said hierarchicalorganization of concepts comprises a set of features, each of saidfeatures being a descendant concept of the root concept of saidorganization, each of said features having as descendants in thetaxonomy a set of concepts, each concept in said set of conceptsrepresenting either a single value or a set of values for said feature;said real estate items are classified, for each said feature, under zeroor more concepts representing either a single value or a set of valuesfor that feature; said step of displaying a reduced taxonomy eitherreports only the concepts belonging to the reduced taxonomy or, for eachsuch concept also reports how many real items in the interest set areclassified under the concept; and said step of pruning of conceptsincludes eliminating from the taxonomy the concepts under which no realestate items in the selected subset of interest is classified, orpreventing such concepts from being selected in order to specifyinterest sets.